Generators are found in virtually every motor vehicle manufactured today. These generators, also referred to as alternators, produce the electricity necessary to power a vehicle's electrical accessories and charge a vehicle's battery. Generators must produce electricity in sufficient quantities to power a vehicle's electrical system. Furthermore, generators must produce electricity having the characteristics necessary to be compatible with a vehicle's electrical components.
A generator includes a stator and a rotor, as seen by reference to U.S. Pat. No. 5,382,862 to Ward et al. In most generators, the stator consists of a metallic core and a current-carrying winding or windings in which electromotive force is induced by a changing magnetic flux. Typically, the core is an annular shaped structure. The internal circumference of the stator is formed with multiple tooth shaped protrusions separated by slots. The current carrying winding, which typically consists of several strands of wire and typically which has three phases, is inserted into the slots and wound around the teeth of the stator.
The generator rotor also typically consists of a metallic core and a current-carrying winding. The rotor current carrying winding typically is made of a single wire wound around the metallic core. The metallic core may be formed with a plurality of claw shaped poles. The poles are situated in pairs such that a pole originating from one end of the core is positioned next to a pole originating from the opposite end of the core. The rotor is rotatably supported at its ends, for example, by bearings installed in the generator housing. The rotor is disposed inside the stator such that the rotor rotates within the annular shaped stator.
Under normal operation, the winding of the rotor is supplied with a current, which induces a magnetic flux in each of the rotor poles. As the rotor rotates, the flux generated at the poles cuts through the current carrying winding of the stator, generating alternating current in it. The alternating current generated in the stator current-carrying winding passes through rectifying circuitry before it is introduced into the electrical system of the vehicle.
The winding pattern of the stator winding and the configuration of stator teeth and slots are significant factors in the generator's operating characteristics. Generator stators typically have one set of current carrying windings, but can have two or more stator windings. Each winding may consist of multiple coils each corresponding to a respective electrical phase p, of which there are typically three. Wires that make up the stator windings are wound into the slots between adjacent stator teeth. Typically, the wire is wound around the stator teeth several times such that bundles of wire are disposed in each slot. The number of stator teeth around which the wire is wound is referred to as the pitch. The windings are typically wound around three stator teeth, which is called a full pitch pattern, and which encompasses 180 electrical degrees. A short pitch pattern is one where the windings are wound around two teeth, and which encompasses something less than 180 electrical degrees. In a full pitch pattern, the wire is guided into a first stator slot, passed over the two slots adjacent to the first stator slot and guided into the fourth stator slot.
As mentioned above, alternating current is generated in the stator windings. Because the vehicle's accessories require direct current, the alternating current generated in the windings must be converted to direct current. The conversion from alternating current to direct current is performed by rectifying circuitry which may include a series of diodes and other electrical circuit elements that are interconnected in a circuit called a bridge. The coils (e.g., coil A, coil B, and coil C for a three-phase stator winding) are conventionally arranged in either a delta or wye configuration.
It is known to construct a generator with two stator windings, as seen by reference to U.S. Pat. No. 5,691,590 to Kawai et al. In dual stator winding generators, the two stator windings are typically spaced from each other in relation to the stator core so that they are wound around different sets of teeth rather than the same sets of teeth. Kawai et al. further disclose that one of the pair of stator windings is offset from the other one of the pair, such that the electricity generated in each winding is shifted in phase by a number of electrical rotational degrees proportional to the distance by which they are offset. Accordingly, Kawai et al. further disclose that two bridge (rectifying) circuits are required, one for each stator winding.
The result of having two bridge circuits is an increase in the number of slots in the stator. Typically, in a two winding stator, if one winding is connected in a wye configuration, then the other is also connected in a wye configuration. Likewise, if one winding is connected in a delta configuration, then the other is also connected in a delta configuration. Therefore, generators with two windings typically include either two wye bridge circuits, or two delta bridge circuits.
Another important design aspect of the stator is the configuration of the stator slots into which the magnetic wire is wound. The number of slots that a stator must have is dependent on the number of rotor pole pairs, n, and the number of electrical phases, p. For a generator having two delta or wye bridges, the number of stator slots is equal to 4×m×n×p, where m is a positive integer, n is the number of rotor pole pairs, and p is the number of electrical phases. Therefore, a generator having two wye or two delta rectifier circuits, six pole pairs, and three phases will require a stator that has 72 slots. There are several disadvantages with conventional generators.
One disadvantage of generators of the type having a single, three-phase stator winding is that they exhibit a significant level of magnetic noise. The addition of a second three-phase stator winding wound around three teeth, without doubling the number of slots/teeth, does not reduce the noise.
Another disadvantage pertains to generators of the type having multiple stator windings. In particular, while doubling the number of stator teeth and adding a second three-phase winding may be effective in reducing magnetic noise, it also increases the number of stator teeth. Stators with an increased number of teeth are more difficult and expensive to manufacture than a stator with fewer teeth.
Still another disadvantage associated with the generators of the type having a stator with two delta windings or two wye windings, is that they require two bridge rectifying circuits, such as disclosed in Kawai et al. referred to above. Two rectifying bridge circuits result in a generator that is more costly and complex than a generator with a single bridge rectifier.
There is therefore a need for a generator that minimizes or eliminates one or more of the problems set forth above.